What is Stray Inductance?

Stray inductance is unintended and unwanted inductance in a circuit.

Inductance does not exist only within inductors. In fact, any wires or component leads that have current flowing through them
create magnetic fields.
When these magnetic fields are created, they can produce an inductive effect. Thus, wires or components leads can act as inductors if they are long enough. Such
effects are often present within circuits (for example, between
conductive runs of wire traces or components with long leads such as capacitors), even though they are not intended. This unintended inductance is referred to as
stray inductance, and it can result
in a disruption of normal current flow within a circuit.

Designers of circuits try to minimize stray inductance as much as possible. They do this by keeping the leads of electronic components
very short and grouping
components in such a way to eliminate capacitive coupling.

A good example of this is a capacitor. When buying a capacitor fresh from
a manufacturer, the leads of the capacitor are usually quite long.

The anode and the cathode leads can extend down for several inches
on capacitors, as can be seen from the illustration above.

However, plugging a capacitor with the leads this long into a circuit can create some issues in the circuit.

When you have capacitors that have very long leads
close to each other, these leads are, essentially, wires. When you have wires close to each
other in circuits, they can produce an inductive effect. Even a small amount of wire can have considerable inductance. When
you have this, the inductance can serve to impede or block high-frequency signals. This is because inductance or a device that acts as an inductor has high impedance
to high-frequency signals. Thus, it is difficult for high-frequency signals to pass through a circuit that exhibits inductive qualities.
When you add unwanted inductance into a circuit, the circuit can now act
(again unwanted) to block high-frequency signals. Thus, a capacitor, which is a device that has low reactance at high frequencies, may now impede high-frequency
signals from passing through. If this is radio circuit or an audio circuit, entire ranges of frequencies may be blocked, which of course can cause poor
transmission of numerous frequencies.
This is why it is desired that no stray inductance should exist in a circuit.

Therefore, this is the reason capacitor leads must be kept short, less than 1.5mm in length ideally, to effectively stop
inductive effects, which can limit a capacitor's
ability to pass high-frequency signals.

Surface mount capacitors are even better to use in circuits because their lead-less terminals being placed directly on the power plane of a
circuit stops almost all
inductance. This is because surface mount components have no leads, but just terminals. This will limit the inductance that capacitors can emit and will allow
them to pass high frequency signals better in a circuit.